Built along the crest of a hill on a SW-NE axis, in the form of an elongated pentagon with pointed E end, and surrounded by a moat. It has an upper bailey to the NE and a lower bailey to the SW. The castle is constructed of substantial blocks of coursed red and grey sandstone. It consists of the following main elements: a substantial curtain wall, battered towards the base and particularly high on the NW (down-hill) side; large round towers to the angles of each bailey; a gatehouse to E with twin D-shaped towers; postern gate to NW curtain wall, in N corner of lower bailey. Internally, the remains of the hall survive to the upper bailey.
The gatehouse to the E consists of 2 projecting D-shaped towers, that to the E surviving to 1 storey, with a pointed-arched doorway to the straight rear wall. The W tower survives up to 2 storeys high, with 2 denuded Tudor-ached openings to the front. Its E side, and the central entrance have been gothicised: The gateway has a wide Tudor arch of stone voussoirs, containing double cast iron gates with scrollwork, flanked by miniature turrets with pierced quatrefoil decoration, over which are crenellated parapets. The flanking walls step down to L and R, behind which a walkway leads over the gateway, reached by stone steps. At right-angles to L, the E wall of the adjoining tower contains a doorcase with 2 octagonal shafts with capitals, supporting a stone cornice, inside which is an arched boarded door; this is flanked by pointed-arched windows, that to R with a diamond grille, boarded behind. The internal side of this wall forms part of a staggered passageway with gothic detail, including decorative cobbled flooring, which leads from the hotel to the gateway steps. Underneath the medieval towers are a series of chambers, probably including prison cells: these are reached by stone staircases, the vaulted roofs supported by chamfered Tudor-arched transverse ribs. Two chambers survive at ground level on the N side of the entrance towers.
The curtain wall running W from the gatehouse joins Ruthin Castle Hotel, its face surviving to the lower parts. It has a small doorway, with Tudor-arched head of stone voussoirs, now infilled. To the NE of the gatehouse, a narrow pointed doorway set at an angle leads to a further section of curtain wall. The NE tower is located at the pointed end of the upper bailey, and survives to a low level; a narrow pointed-arched doorway leads into it. Running towards the W, the curtain wall is better preserved and contains angled slits, including red sandstone blocks with angled oval openings. Turning towards the SW, the high curtain wall has 3 round towers along its length. The NE tower survives about 1m above ground level and contains the base of a winding staircase. The hall is located between the NE and central towers, a large rectangular building with high wall to NW, and low wall to SE. The end walls are not well-preserved, but there is a narrow Tudor-arched doorway to N corner. Between the NW wall and curtain wall was a narrow corridor, possibly containing service areas. It has tall round-arched doorways with red sandstone dressings to each end, and pointed-arched window openings to the curtain wall. However, the NW wall of the hall was gothicised, with crenellated parapets and stepped buttresses to the inner (hall) side. The corridor appears to have been re-used as a garden structure, possibly a hot-house. It had 2 fireplaces with stone lintels to the inner wall, now infilled with brick, recessed arches to both sides, and evidence for a lean-to roof. To the SW, the wall of the central tower stands c1.2m high and has recesses for 4 window openings. Adjacent to this is a probable wheel-pit, with leat in N corner which could have provided water for an overshot wheel. This structure could have been associated with the castle entrance, a mill within the castle, or a C19 garden feature. Beyond the central tower is the lower bailey, the ground level falling sharply, and with spiral and straight stairs leading down. At the bottom, the curtain wall contains a low postern gate (visible from outside), and a gateway to its SW, a wide archway of stone voussoirs, with grooves for a portcullis; there is evidence for a tower structure above this. The curtain wall was partly damaged during the Civil War. The SW tower was rebuilt in red sandstone in the mid C19, and forms part of the formal gardens of Ruthin Castle Hotel. A wall runs SE from the tower and then terminates.